1. Field of the Invention
This invention relates to a television and its back lighting source module, more particularly to a television and a back lighting source module capable of preventing harmonic interference.
2. Description of the Related Art
Flat Panel Display (FPD) devices, which are used as liquid crystal televisions, plasma televisions, etc., are advantageous over conventional Cathode Ray Tube (CRT) devices in that they generate less radiation and are thinner, lighter, and more environmental friendly. Therefore, it has become a current trend to replace conventional CRT televisions with FPD televisions.
A conventional LCD television includes a display panel and a back lighting source module. The display panel includes components, such as conductive glass, liquid crystals, orientation films, color filters, polarizers, drive control elements, etc. The back lighting source module includes components, such as a light source (i.e., cold cathode lamps or light emitting diodes), a light guide plate, various optical films, etc. Moreover, an inverter enables the back lighting source module to generate a source control signal for controlling turning on and turning off of the light source.
As shown in FIG. 1, adjustment of brightness and contrast in an LCD TV is generally realized through adjustment of the amplitude (a) of a power signal 101 applied to the light source. In actual measurements, adjustment of the amplitude (a) between maximum and minimum values only results in a narrow range of adjustable brightness values, i.e., between 250 cd/m2 and 180 cd/m2, for the LCD TV.
Referring to FIG. 2, an LCD TV capable of operating in a burst mode is shown to include a light source control unit 3 and a set of light sources 34 (such as lamp tubes). The light source control unit 3 includes a power circuit 30, a control unit 31, a transformer unit 32, and a feedback unit 33.
Unlike the control scheme of FIG. 1, when the amplitude of the power signal is adjusted to the minimum value, the control unit 31 operates in a burst mode, where the control unit 31 receives a width-adjustable square-wave signal 301 and sends out a source control signal 201 to control the light sources 34 through the transformer unit 32 and the feedback unit 33 according to the square-wave signal 301.
In the burst mode, power to the light sources 34 is interrupted intermittently according to the square-wave signal 301. As shown in FIG. 3, the time duration (D) of the source control signal 201 from the control unit 31 in which the light sources 34 are turned on and turned off continuously is so controlled to cause persistence of vision such that a visual lowest brightness output of the light sources 34 is obtained, whereas the time duration (d) corresponds to the turn-off period of the light sources 34. By adjusting the length of the duration (d) to the duration (D), a wider range of adjustable brightness values, for example, between 250 cd/m2 and 50 cd/m2, is achieved.
However, during the process of adjusting the source control signal 201, if the resulting turn-on/turn-off frequency of the light sources 34 happens to be an integer multiple of a display scanning frequency, this arises in harmonic interference, which will result in a mura phenomenon on the screen of the LCD TV, thereby affecting adversely the quality of images presented by the LCD TV. A similar mura phenomenon also happens in back-projection televisions and LCD projector devices.